Why does UnderscoreJS use toString.call() instead of typeof?

Question

Looking under the hood in UnderscoreJS, I see:

  _.isFunction = function(obj) {
    return toString.call(obj) == \'[object Function]\';
  };

  _.isString = func         


        

Answer 1

Well actually this is because it is faster to check the [[Class]] by checking with toString. Also there could be less mistakes, since toString gives you the exact Class ...

check this :

var fn = function() { 
    console.log(typeof(arguments)) // returns object
    console.log(arguments.toString()) // returns object Arguments
}

You could see the benchmark for underscore typeof vs toString here :

http://jsperf.com/underscore-js-istype-alternatives

Also there are some github issues with better explaination :

https://github.com/documentcloud/underscore/pull/332

https://github.com/documentcloud/underscore/pull/321

EDIT 1 :

You could also check this great article :

http://javascriptweblog.wordpress.com/2011/08/08/fixing-the-javascript-typeof-operator/